The compound (E)-4-bromo-2-methylbut-2-en-1-al is known in the art and is important for its use as a starting material for (E)-4-acetoxy-2-methylbut-2-en-1-al a key intermediate in one of the industrial synthesis of Vitamin A and Vitamin A acetate. Moreover, the compound can also be used as an intermediate in the synthesis of various other biologically active compounds containing isoprene derived groups.
It has heretofore been proposed by G. Eletti - Bianchi et al., J. Org. Chem., 41, 1648-50 (1976) to prepare the corresponding chlorinated derivative of isoprene epoxide by chlorinating isoprene epoxide with cupric chloride in the presence of lithium chloride as a catalyst. A similar disclosure is to be found in U.S. Pat. No. 4,054,608 issued Oct. 18, 1977, to Luciano Re et al. and assigned to Anic S.p.A.
In the article by G. Eletti - Bianchi et al. the authors indicate that the synthesis of (E)-4-chloro-2-methylbut-2-en-1-al from 3,4-epoxy-3-methyl-1-butene and copper chloride is accomplished through a mechanism wherein there is a nucleophilic attack of a chloride ion on the terminal olefinic carbon in 3,4-epoxy-3-methyl-1-butene. However, since a bromide ion is a much poorer nucleophile than the chloride ion, copper bromide would not be expected to be sufficiently reactive to be utilized in such a process. Moreover, since the reaction produces a hydrohalic acid and the desired product of the present application, namely (E)-4-bromo-2-methylbut-2-en-1-al is acid sensitive, it would either decompose in the presence of the hydrobromic acid or further react therewith. Thus, it is readily seen that the prior art procedure disclose for preparation of the chlorinated derivatives is not suitable for the preparation of the corresponding brominated derivative and a different process or process conditions are required to obtain (E)-4-bromo-2-methylbut-2-en-1-al and to obtain good yields of said product.
It has also been known that 4-bromo-2-methylbut-2- en-1-al can be obtained from a reaction mixture of hydrobromic acid, 1,1-dimethoxy-2-methylbut-3-en-2-ol, toluene, sodium bromide and cupric bromide, for example, as disclosed in Example 3 of U.S. Pat. No. 3,940,445 issued Feb. 24, 1976, to Werner Reif et al. and assigned to Badische Anilin-& Soda-Fabrik Aktiengesellschaft. However, the yield obtained from said reaction is not acceptable and a more productive method is highly desired. Also, the starting materials are extremely difficult to prepare and the reaction is not stereospecific.